The objective of this research is to relate the topographic and axon collateral organization of thalamocortical and corticocortical projections to tonotopic maps in cat auditory cortex. This is achieved using a double retrograde axonal transport technique in combination with electrophysiological mapping of auditory cortex. Horseradish peroxidase (HRP) and tritiated bovine serum albumin (3H-BSA) are each retrogradely transported from axon terminals to cell bodies, and neurons containing each substance are mutually distinguishable. Injection of HRP and 3H-BSA into different cortical regions in the same experiment allows identification of neurons which project to one or the other region or to both regions via axon collaterals. Injection of the two tracers into different tonotopic cortical fields allows mapping of the locations of thalamic and Al cortical neurons which project to each of these fields. Quantitative estimates will be made of the percentage of neurons projecting to one field which also project to each of the other fields. Additionally the topography of thalamic projections will be related to best-frequency gradients in fields P and VP, and the topography of thalamic projections and cortical projections will be related to isofrequency strips in different fields. Finally, the presence of auditory thalamic and cortical neurons with axon collaterals projecting to three different tonotopic fields will be investigated using a retrograde fluorescent labeling technique.